Look, here are the reasons why inbreeding is not common in Naruto.
1) First and foremost, Naruto isn't like other anime, and anime doesn't present a close outlook of Japanese culture like Naruto does, to an extent. In Japan, you won't find stuff like incest, gay obsessiveness or schoolgirls with huge boobs with their skirts so high you can see their 'panties' when they bend over everywhere.
2) In Ancient Japan, amongst royalty, incest would take place. Since Naruto is in a time with radios and computers, and the Uchiha are only shinobi, incest isn't taking place.
3) Since Hashirama's era, genetic strength has dwindled greatly (excluding the exceptions). If inbreeding was common, the decline rate would be a lot slower and shinobi would retain a lot of their power. Plus, shinobi would be a lot shorter at this point.
And yes Snape, she was an Uchiha by marriage.